1. Field of the Invention
The present invention generally relates to a method for growing III-V group compound semiconductor crystal, and more particularly to a method for growing III-V group compound semiconductor crystal in which the uniform silicon doping is available at high concentration at low temperature.
2. Description of Prior Art
Currently, III-V group compound semiconductors are being used in a wide variety of fields because of a high speed operation due to a characteristic band structure which provides a reduced effective mass of electrons. A typical example of such a III-V group compound semiconductor device is a HEMT (High Electron Mobility Transistor). The HEMT includes an undoped channel layer and an n-type electron supplying layer provided on the channel layer. In such a structure, 2DEG (2 Dimensional Electron Gas) is formed in the channel layer along an interface to the electron supplying layer. By interrupting the 2DEG by means of a gate electrode, it is possible to switch the device on and off.
In this type of semiconductor device, the threshold voltage (V.sub.th) for turning on and off the device is determined generally by a thickness of the electron supplying layer (t) and the carrier density thereof (N.sub.d) according to the following equation: EQU V.sub.th =cN.sub.d t
c: constant.
Accordingly, controlling the thickness of the electron supplying layer and the carrier density is important. In order to form the semiconductor layer, a vapor-phase growth method is normally used.
When silicon (Si) is used as a dopant in a III-V group compound semiconductor crystal in this method, silane gas (SiH.sub.4) or disilane gas (Si.sub.2 H.sub.6) is conventionally used as the silicon material. However, two problems exist in the conventional method. One is a safety problem in handling the gases and the other is a high decomposition temperature of the gases.
As for the safety problem, as these gases are combustible and are normally stored in a tank at a high pressure, the gases may possibly explode and lead to a disaster due to a defect in the tank or an operation mistake. Accordingly, these gases have been controlled as a special high-pressure gas in the Japanese High Pressure Gas Control Act since 1992 in Japan. On the other hand, as a compound similar to the silicon compound in characteristics, arsine (AsH.sub.3) or phosphine (PH.sub.3) can be listed. However, since these compounds are poisonous, research on a substitute for the compounds has been conducted. t-butylarsine (C.sub.4 H.sub.9 AsH.sub.2) or t-butylphosphine (C.sub.4 H.sub.9 PH.sub.2) can be listed as the substitute.
However, as long as the Si material is a high-pressure combustible gas, there must be a countermeasure for the operation safety assuming an explosion even if the V group compound is substituted with a safe material.
As for the decomposition temperature problem, the bond energy of Si--H is relatively high and the decomposition rate thereof is low in Si material. Accordingly, disilane, which has a lower decomposition temperature than silane, is normally used when the doping is conducted at a high concentration of more than 10.sup.18 cm.sup.-3. However, as a substrate and a growth furnace for the method are increased in size, a pressure at which vapor-phase growth is done is lowered in order to improve a uniformity of doping. Therefore, a dependency of doping efficiency on growth temperature cannot be disregarded, and it is difficult to improve a uniformity of donor concentration on the large-size substrate. The reason for the difficulty is that when the decomposition temperature is high and the crystal is grown at a reduced pressure, the decomposition rate is determined by the decomposition reaction rate.
On the other hand, when the decomposition temperature is low, the material is decomposed in the vapor phase. That is, the decomposition rate is determined by the diffusion rate of the material. In this case the uniformity of the donor concentration can be easily improved. One example having such characteristics is III group compound.
In order to realize an accurate process at a high concentration, for example, a non-alloy ohmic process, in which InGaAs doped with Si is grown at a high concentration equal to or higher than 10.sup.19 cm.sup.-3 on the surface of a grown layer, is conducted. However, in this process, when InGaAs is grown on a GaAs substrate, the crystal surface becomes rugged and white due to the heterogeneity of lattice, which leads to polycrystalization. According to the latest research, it is necessary to extremely lower the growth temperature to under 500.degree. C. in order to prevent the polycrystalization. However, as disilane is not decomposed at such a low temperature, the doping cannot be realized at a high concentration of more than 10.sup.19 cm.sup.-3 unless an excess amount of disilane is introduced into the reaction tank.
If such an excess amount of Si doping material is introduced into the reaction tank, Si doping material may attach to the tank wall and remain in the tank. This produces a memory effect in the tank. Conventionally, it has been reported that Si doping material does not have the memory effect. This may be because the amount of the Si doping material used in the reaction was relatively less. Considering the vapor pressure of a Si element, when an excess amount of the Si doping material is used, the memory effects may possibly appear, just as for selenium (Se) and zinc (Zn) doping material in which memory effects have been reported.
In order to solve above-mentioned two problems, tetramethylsilane and hexamethydisilane, organic silicon compounds which are liquid at room temperature are disclosed in Japanese Laid-Open Patent Application No. 3-280419. However, if every hydrogen atom in a Si material is substituted by an alkyl group, the decomposition energy is increased. Accordingly, the decomposition temperature of the Si material is increased, and the resulting material is more difficult to handle than silane. The reason for this difficulty is considered to be a difference in characteristics between hydride of V family material and alkyl derivatives thereof. Also, when the material having only Si--C bonds is used, carbon atoms can be easily mixed into the acceptance site of the crystal and reduce the donor concentration.
As discussed above, when silane or disilane, which are the special combustible gas, is used, a big explosion may occur as the gas is stored at the high pressure in the tank.
Also, since the decomposition temperature of these gases is high, it is difficult to improve the uniformity of the doping efficiency on the large-size substrate.
Further, as the decomposition temperature of the above-mentioned compounds is relatively high, a large amount of Si material must be introduced in order to realize the high concentration doping at the low temperature. Therefore, the memory effects of the Si material in a reaction tank must be considered.
Moreover, the proposed substitute for silane has the very high decomposition temperature and has the structure into which carbon is easily mixed with Si. A substitute which satisfies the necessary characteristics to solve the above problems has not been proposed so far.